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Unformatted text preview: Chapter 16 Mechanisms of Heat Transfer 16-1 Chapter 16 MECHANISMS OF HEAT TRANSFER Heat Transfer Mechanisms 16-1C The house with the lower rate of heat transfer through the walls will be more energy efficient. Heat conduction is proportional to thermal conductivity (which is 0.72 W/m. ° C for brick and 0.17 W/m. ° C for wood, Table 16-1) and inversely proportional to thickness. The wood house is more energy efficient since the wood wall is twice as thick but it has about one-fourth the conductivity of brick wall. 16-2C The thermal conductivity of a material is the rate of heat transfer through a unit thickness of the material per unit area and per unit temperature difference. The thermal conductivity of a material is a measure of how fast heat will be conducted in that material. 16-3C The mechanisms of heat transfer are conduction, convection and radiation. Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas which is in motion, and it involves combined effects of conduction and fluid motion. Radiation is energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules. 16-4C In solids, conduction is due to the combination of the vibrations of the molecules in a lattice and the energy transport by free electrons. In gases and liquids, it is due to the collisions of the molecules during their random motion. 16-5C The parameters that effect the rate of heat conduction through a windowless wall are the geometry and surface area of wall, its thickness, the material of the wall, and the temperature difference across the wall. 16-6C Conduction is expressed by Fourier's law of conduction as & Q kA dT dx cond = - where dT/dx is the temperature gradient, k is the thermal conductivity, and A is the area which is normal to the direction of heat transfer. Convection is expressed by Newton's law of cooling as ) ( ∞- = T T hA Q s s conv & where h is the convection heat transfer coefficient, A s is the surface area through which convection heat transfer takes place, T s is the surface temperature and T ∞ is the temperature of the fluid sufficiently far from the surface. Radiation is expressed by Stefan-Boltzman law as ) ( 4 4 surr s s rad T T A Q- = es & where e is the emissivity of surface, A s is the surface area, T s is the surface temperature, T surr is average surrounding surface temperature and s = ×- 567 10 8 . W / m .K 2 4 is the Stefan-Boltzman constant....
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